This invention relates to ceramic celluar structures and more particularly relates to such structures having a high cell density per unit area and to a method for producing such structures and furthermore relates to the use of such structures regardless of cell density as heat exchangers and catalyst supports.
The attractiveness of ceramics as materials for cellular structures to support catalyst materials and to act as heat exchangers is widely recognized. For example, the heat exchanger is an integral part of gas turbine engine designs. The purpose of the heat exchanger is to recover waste heat losses and to preheat the incoming air in order to improve the efficiency of engine operation. The level of both fuel consumption, and noxious exhaust emissions are expected to be significantly lower than for conventional internal combustion engines. In one design, the heat exchanger is a slowly rotating device, (regenerator), heated by hot exhaust gases on one side and giving up this heat during rotation to the incoming cool gases on the other side. In another design the heat exchanger is stationary (recuperator), and hot and cold gases are passed through alternate layers. Whether in the form of a rotary regenerator or a stationary recuperator ceramic heat exchangers have the advantage of high termperature capability which has a direct effect on engine efficiency. Ceramics are also lighter in weight than metals and have the potential for lower cost.
A major disadvantage of ceramics is the difficulty inherent in forming them, due in large part to their low strength in the green, unfired state and to their brittleness in the fired state. This problem becomes more acute as surface area and size requirements for these structures increase, requiring decreased cell wall thicknesses and increased bulk weight.
One approach to solving such forming problems has been to cast a ceramic film from a slurry onto a fugitive support material to form a bilayered tape, mold the bilayered tape into a corrugated member, form the tape into the desired structure (for example, by rolling or stacking with interposing flat members) and fire to volatilize the support medium and sinter-weld the structure.
This technique of utilizing a fugitive support material to provide needed strength during forming imposes an upper limit on cell density due to the space occupied by the support layer in the structure prior to firing. In addition, where the wall thickness is small compared to the support thickness, substantial contact of the nodes of the corrugated layer may be prevented (particularly in a rolled structure) resulting in formation of few sinter welds during firing and consequent low mechanical strength of the finished structure.
In a similar approach the support is of a material such as aluminum which upon firing converts to the oxide and thus becomes an integral part of the structure. Unfortunately, such an approach seriously limits the compositional choices of the ceramic particularly in high temperature applications where the thermal expansion coefficient of the structure is of critical importance. In gas turbine applications, for example, where the ceramic heat exchanger would be subjected to severe thermal shocks, both the ceramic and the support material would have to exhibit high thermal shock resistance and thus high thermal expansion compatibility.
While U.S. Pat. No. 3,444,925 does describe a technique for fabricating such cellular structures whereby use of the support material is optional, the structure described having the highest cell density (about 500 cells per square inch) was achieved with an aluminum foil support.
It is therefore felt that a ceramic based composition which can be processed to maintain sufficient flexibility in the green state to be molded in small thicknesses, yet retain sufficient green strength to maintain its molded shape prior to firing without the need of a substrative support material and which would thus enable the fabrication of high cell density cellular structures would be an advancement in the art.